The invention also relates to a method of manufacturing a plurality of such sensors using semiconductor material micro-machining technologies.
The pressure measurement sensors, made of semiconductor based materials, most commonly used to measure absolute pressure are silicon sensors of the piezo-resistive type.
These piezo-resistive sensors generally comprise a silicon membrane in which are incorporated piezo-resistive elements, that is to say elements whose resistivity varies according to the pressure to which they are subjected.
These sensors, although having a very simple structure and small dimensions, present a number of disadvantages.
Their lack of temperature stability necessitates the use of a temperature compensating circuit in order to obtain an accurate measurement. Added to this are reactions such as interdiffusions between the silicon of the membrane and the piezo-resistive elements, which accelerate the aging process of the sensors.
This is why, when the use of semiconductor sensors requires durability, temperature stability, very high sensitivity and low consumption, capacitive sensors are generally used to measure absolute pressure.
Capacitive sensors generally comprise a first element in which is made a membrane forming a mobile electrode and a second element in which is arranged a zone forming a fixed electrode, known as a counter-electrode. An insulating frame is interposed between the first and the second elements to create a chamber, insulated from the external environment, inside which there is zero pressure. A capacitive absolute pressure sensor is thus produced as the external pressure is measured in relation to the zero pressure, or almost zero pressure, prevailing inside the chamber.
One disadvantage of these sensors arises from the technology for the manufacture of the latter, in this case the semiconductor material micro-machining technology.
In the course of manufacture it is noted that degassing can occur within the structure of the sensor leading to the creation of residual pressure inside the chamber. The sensor does not, thus, under these conditions give an exact reading of the absolute pressure. Furthermore, the volume of gas contained in the chamber varies according to the temperature, so that the stability and/or the reproductibility of the readings are affected.
A known solution to eliminate this problem consists of placing a cavity in the first element in order to create a so-called reference volume. The object of this reference volume in connection with the chamber volume is to reduce the residual pressure caused by the degassing.
However, this solution in itself presents disadvantages.
When the reference volume is located outside the active surface of the membrane, that is to say outside the surface of the sensor which is subjected to pressure, the total surface of the sensor is increased by an amount corresponding to the surface occupied by the reference volume so that, for sensors providing active surface data, the number of realizable sensors on the same wafer of silicon is reduced, which in itself increases the cost of each sensor.
Further, the inclusion of the reference volume in the first element requires machining the latter on two faces which complicates the process considerably.
As these sensors are manufactured in batches from silicon substrate, at the end of the process they must be separated from each other in the course of a cutting operation across the first and second substrates. To this end, it is necessary to provide a supplementary surface at the periphery of the membrane which also reduces the number of sensors which can be produced on the surface of a wafer. Further, this cutting is generally by mechanical sawing, which causes stresses, or even cracks, within the membrane of the sensors, which generally lead to irreparable damage to the sensors.